Patent GB-677984-A (GB)
Title: Improvements in or relating to telecommunication electric pulse systems
Inventor: REEVES ALEC HARLEY
IPC: H04Q11/04
Filed: 1947-07-22
Abstract: 677,984. Automatic exchange systems. STANDARD TELEPHONES & CABLES, Ltd. July 16, 1948 [July 22, 1947], No. 19615/47. Class 40 (iv). [Also in Group XL (c)] In a pulse modulation transmission system which may be employed in a telecommunication exchange, pulse times are selected from a first pulse repetition time cycle and the modulations carried by these selected pulse times are transferred to a corresponding number of pulse times in a second independent and unrelated pulse repetition time cycle. Multiplex pulse exchange system.- Fig. 1 shows the system applied to a telecommunication exchange wherein line-finder units 1, 2, 3 are connected to a looped line 27, 25. Delay networks 4, 5, 6 are connected in line 27 between line-finder units 1, 2, 3, and delay networks 8, 9 and 10 between the connections to each line-finder unit. Rectifiers 11, 12, 13, are also included in line 27. Each line-finder unit consists of a gate circuit 16, a selector 17, a demodulator 18 and a pulse generator 20. Each group of line-finders serves a number of substations having their signals applied to terminal 7 in the form of time modulated pulses of amplitude level " A." At the commencement of a call from a substation however, a pulse of greater amplitude level " B " is transmitted and applied via terminal 7 to the first free gate circuit 16 of a line-finder unit, the engaged gate circuits or selectors 17 already in operation rejecting the pulse. The gate circuit 16 is arranged so that after being opened by a level B pulse it continues to respond to level A pulses whilst the selector 17 rejects all channels other than that of the particular substation making the call. Pulse generator 20 is arranged so that on receipt of a level B pulse only from selector 17 it generates a pulse of amplitude equal to the difference in amplitude between a level A and a level B pulse. This pulse is applied to line 27 at the point 21 so that it arrives at the same time as that part of the level B pulse passing through delay network 8 and rectifier 11 and so that a level A pulse then passes down line 27 which has no effect on subsequent line-finder units. Subsequent level A pulses arriving via the gate circuit 16 and selector 17 at the demodulator 18 are demodulated by means of master pulse applied at terminal 19, the demodulated signal being passed to a cathode-ray distributer modulator 15, the output being taken from terminal 23. A large number of substations may therefore be dealt with by a small number of channels. Release is effected by means of a pulse of level C, less than level A, which fails to keep open the gate circuit 16. The next level B pulse will then travel round the line until it reaches the first free linefinder. In the event of all the line-finders being already in use the level B pulse is returned via the portion of the line 25 to a device 14 which is arranged to generate a level B pulse if either (a) the incoming pulse on terminal 7 is a B pulse, or (b) the pulse being returned along line 25 is a B pulse. A pulses are otherwise generated, whilst any C pulse returning via line 25 after freeing a line-finder is transformed into a level A pulse by an amplifier 26 so as not to debar the subscriber from making a further call. Gate and selector circuits.-The gating and/or circuits of the line-finder units may be as shown in Fig. 2, employing a gas-filled cold cathode diode 33. A battery 37 is connected across the tube so that it will not fire until the arrival of a pulse at terminal 40 at time 50 (Fig. 5). The tube fires causing the voltage at the cathode 35 to rise from zero to level 53 (Fig. 6). The rise in voltage extinguishes the tube and the voltage on the cathode falls exponentially to a level 65 at time 51 when the next channel pulse arrives and again fires the tube. The tube cannot be fired again until after time 55 when the cathode voltage has fallen to level 54 so that it is not fired by pulses of other channels. In modifications, in order to make the slope of the decay curve greater and permit a larger number of channels to be used, an inductance is included in the cathode circuit of the diode, Figs. 3 and 7 (not shown), or a delay network may be included between the cathode and earth. The delay network is such that it returns a pulse of negative polarity to the cathode circuit, thus lowering its voltage at the instant the next required channel pulse is received on terminal 40 (Figs. 4 and 8, not shown). If the circuits are used as gate circuits in addition to selective circuits they are made to respond initially only to a level B pulse and thereafter to level A pulses until the arrival of a level C pulse which fails to fire the tube. The tube will then require a level B pulse to be restarted. Multi-gap cold cathode gas tubes of the type described in Specifications 665,817, [Group XIX], 665,818, 665,820 and 677,985, [all in Group XL (c)], may also be used as gate and selector circuits 16, 17. Specifications 581,613, 624,017, [Group XXXV], 632,375, 632,376, 650,032, 659,805, [both in Group XL (c)], and 665,819 also are referred to.